Effects of biological production and vertical mixing on sea surface pCO2 variations in the Changjiang River plume during early autumn: A buoy‐based time series study

A month‐long, high‐resolution buoy time series from the surface ocean of the Changjiang River plume in early autumn 2013 (30‐min sampling frequency) show great variability in the partial pressure of carbon dioxide (pCO2), and other physical and biogeochemical parameters. Early in the deployment, surface pCO2 decreased by ~117 μatm in a single day (11–12 September, from an initial value of ~527 μatm); a similar decline of 62 μatm occurred 5 days later (to ~378 μatm). Both drawdown events were associated with strong vertical stratification, high chlorophyll a concentrations, and oxygen supersaturation. A one‐dimensional mass balance model suggests that biological production was responsible for more than half the pCO2 decrease observed during 10–23 September. Subsequently, in association with strong winds, the mixed layer rapidly deepened and surface pCO2 increased sharply (by about 108 μatm in late September and again in early October). Vertical mixing accounted for more than half of this pCO2 increase, which offset more than the earlier biologically‐driven CO2 drawdown. In the presence of such strong temporal variations of pCO2, sampling frequency exerts a substantial influence on air‐sea CO2 flux calculations for the Changjiang River plume and similar coastal areas. Compared to daily sampling, even weekly sampling would result in a bias of up to ±4.7 mmol C m−2 d−1, or ±63% error.

Li D., Chen J., Ni X., Wang K., Zeng D., Wang B., Jin H., Huang D. & Cai W.-J., in press. Effects of biological production and vertical mixing on sea surface pCO2 variations in the Changjiang River plume during early autumn: A buoy‐based time series study. Journal of Geophysical Research: Oceans. Article (subscription required).

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